Devices using spin-momentum transfer, such as those described in U.S. Pat. No. 5,695,864 (Slonczewski) and J. Slonczewski, Journal of Magnetism and Magnetic Materials 159, L1 (1996), consist of at least two ferromagnetic regions, which play distinct roles. One of these regions is designed to have its magnetization fixed or pinned while the other region has a changeable magnetization direction. The fixed magnetization region serves two functions: it both provides a spin polarized current and it serves as a reference layer to detect changes in the magnetization of the second region. The changes in magnetization direction of the second region lead to changes in device resistance, which can be detected electrically. The origin of this change in resistance is giant magneto-resistance or tunnel magnetoresistance. For MRAM applications one of the key challenges is to reduce the current and power needed change the magnetization state and induce magnetization precession. A further challenge is to have a significant change in resistance so as to have a large device output signal. Thus methods, operating principles and materials are sought which enable device operations at lower currents and large device output signal.
In 1996, a new mechanism was proposed by which the magnetization of a small ferromagnetic element may be manipulated when the latter is traversed by a spin polarized charge current. This concept, known as spin transfer, relies on the local interaction between a spin polarized current and the background magnetization of a ferromagnet. Spin transfer induced magnetization dynamics has been recently demonstrated experimentally in a wide variety of the material systems and is rapidly reaching device maturity. Furthermore, direct measurements of the magnetization dynamics have shed light on the fundamental mechanism of spin transfer.
With a few exceptions, most spin transfer devices consist of at least two exchange decoupled ferromagnetic regions, which play distinct roles. One of these regions is designed such that it simultaneously provides both a spin polarized current and a reference layer relative to which the manipulation of the second ferromagnetic region is detected. However, recently it has been demonstrated that such a distinction between a “fixed layer” and a “free layer” is not necessary. In large magnetic fields applied perpendicular to the thin film plane, current induced reversible changes in junction resistance have been observed in pillar junctions having only a single ferromagnetic layer. These excitations have been associated with the onset of non-uniform spin wave modes. The spin transfer mechanism relies on a feedback mechanism between the single layer magnetization and the spin accumulation in the adjacent nonmagnetic layers.
In magnetic fields below the demagnetization field, spin transfer may lead to both reversible excitations and hysteretic changes in the junction resistance. Systematic, abrupt and hysteretic changes in device resistance have been observed in sub-100 nm size pillar junctions containing a single ferromagnetic (FM) layer in small perpendicular magnetic fields (H<Hdemag). Both the threshold currents and the current induced hysteresis in these junctions exhibit a well defined magnetic field dependence. Similar to the high field excitations, the low field single layer switching events lead to a low resistance state. However, the threshold currents and the corresponding changes in device resistance in the two regimes are very distinct. Remarkably, in the low field regime the magnetoresistance is comparable to that observed in the more traditional bilayer geometry. Recent micromagnetic studies discuss considerable inhomogeneities of the free layer magnetization during magnetization reversal in bilayer junctions.